Fix nil pointer dereference in supervisor example (#166)

`Supervisor.healthCheckTicker` is prepared in `(*Supervisor).startAgent`. If there is no effective config file, we will skip the call of `startAgent` causing an nil pointer dereference in the loop.

internal/examples/supervisor/supervisor/supervisor.go

@@ -475,6 +475,11 @@ func (s *Supervisor) runAgentProcess() {
 	restartTimer.Stop()
 
 	for {
+		var healthCheckTickerCh <-chan time.Time
+		if s.healthCheckTicker != nil {
+			healthCheckTickerCh = s.healthCheckTicker.C
+		}
+
 		select {
 		case <-s.hasNewConfig:
 			restartTimer.Stop()
@@ -498,7 +503,7 @@ func (s *Supervisor) runAgentProcess() {
 		case <-restartTimer.C:
 			s.startAgent()
 
-		case <-s.healthCheckTicker.C:
+		case <-healthCheckTickerCh:
 			s.healthCheck()
 		}
 	}

internal/examples/supervisor/supervisor/supervisor_test.go

@@ -0,0 +1,64 @@
+package supervisor
+
+import (
+	"fmt"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/open-telemetry/opamp-go/internal"
+	"github.com/open-telemetry/opamp-go/internal/examples/server/data"
+	"github.com/open-telemetry/opamp-go/internal/examples/server/opampsrv"
+)
+
+func changeCurrentDir(t *testing.T) string {
+	t.Helper()
+
+	tmp := t.TempDir()
+
+	oldCWD, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("getting working directory: %v", err)
+	}
+
+	if err := os.Chdir(tmp); err != nil {
+		t.Fatalf("changing working directory: %v", err)
+	}
+	t.Cleanup(func() {
+		if err := os.Chdir(oldCWD); err != nil {
+			t.Fatalf("restoring working directory: %v", err)
+		}
+	})
+
+	return tmp
+}
+
+func startOpampServer(t *testing.T) {
+	t.Helper()
+
+	opampSrv := opampsrv.NewServer(&data.AllAgents)
+	opampSrv.Start()
+
+	t.Cleanup(func() {
+		opampSrv.Stop()
+	})
+}
+
+func TestNewSupervisor(t *testing.T) {
+	tmpDir := changeCurrentDir(t)
+	os.WriteFile("supervisor.yaml", []byte(fmt.Sprintf(`
+server:
+  endpoint: ws://127.0.0.1:4320/v1/opamp
+agent:
+  executable: %s/dummy_agent.sh`, tmpDir)), 0644)
+
+	os.WriteFile("dummy_agent.sh", []byte("#!/bin/sh\nsleep 9999\n"), 0755)
+
+	startOpampServer(t)
+
+	supervisor, err := NewSupervisor(&internal.NopLogger{})
+	assert.NoError(t, err)
+
+	supervisor.Shutdown()
+}